The exacting application of torque is a requirement during all phases of oilfield drilling and completion. In response, many vendors employ a variety of methods to apply calibrated rotary motion to achieve desired levels of torque. With respect to calibration in general, typically a device that measures a linear dimension is calibrated against a certified length standard, a pressure gauge against a calibrated pressure gauge, and so forth, where calibration may be defined as the process of adjusting the output or indication on a measurement instrument to agree with the value of the applied standard. Calibration standards, in turn, are similarly calibrated against even more precise standards and so on until the reference is a national standard. A chain of authority is created such that the lowest link can refer up through cascading standards to a singular standard.
The calibration process with respect to torque measurements is largely disregarded in oilfield applications. Torque measuring devices are not typically calibrated by the application of a known force. Instead, load cells are used to measure torque referentially (as opposed to directly), and these load cells are calibrated by the application of force with no involvement of torque. In the United States, torque is typically measured in the number of pounds applied to a one foot long moment arm. To better understand why torque measurement is subordinated, perhaps an examination of pressure measurement would be informative. Pressure standards, known as dead weight testers, directly generate calibrated loads in pounds per square inch (psi). The load is generated by the application of a known weight on a piston of known diameter. Knowing the weight and the cylinder diameter enables the accurate calculation of the hydrostatic load measured in psi. A hierarchy of dead weight standards of ever increasing accuracy culminating with the national standard are available as desired.
Unfortunately, torque measurements do not lead to such straightforward solutions. There are no recognized national torque standards. Thus, torque measurements are made by indirect reference. Typically, oilfield processes measure torque referentially by the torque reaction of a measured reaction arm against a calibrated pressure sensor or mathematically by the application of a measured amount of electrical energy to a motor attached to a gearbox with a known gear reduction. Too often these referential torque measurements are made far away from the object of interest, in particular oilfield tubular connections. These tubular connections have precise torque requirements and often specify torque tolerances of only 10% away from nominal. Despite the best efforts of service providers, torque measurements often have significant errors, far exceeding the 10% allowance specified by connection suppliers.
In oilfield environments, electronic load cells are most often the source of data, and as such are frequently calibrated to 1% accuracy, for which there is no dispute with respect to the calibration method. The installation of load cells, however, is open to substantial criticism. The moment arm in this case is measured with a tape measure by identifying the center of rotation of a tong to the clevis attached to the tong. The snub line attached to the tong is either to be 90° from tong body or of a known angle. So far, it is easy to imagine a variety of errors that can affect the torque measurement, including arm length errors and snub line angle errors (in two planes).
Even assuming all the measurements are precise, yet another more insidious error is introduced: unknown, asymmetric, and spurious parasitic torque losses developed by the tong. Despite the best efforts of measuring the reaction torque of the tong body, the measurements do not quantify the actual torque applied to the connection of interest. In this case, only the application of torque by the use of a pipe tong is examined. Known methods of torque application suffer significant errors in torque measurement through faulty mechanics, such that these measurements suffer significant parasitic torque losses, the errors are not symmetric, and ultimately the torque measured has only a distant relationship with the torque applied.
Thus, there exists a need for a device that can measure torque in-situ regardless of its physical orientation.